September 20, 2013

The problem with Cube-style Hybrid 3d printer/mill combos.

I’ve noticed a plethora of Cube-style Hybrid 3d printers/mills coming onto market about now. Having designed a Hybrid printer before, I can tell you why I didn’t decide to go with a cube. Not building a cube is suicide — Cubes sell for more than non-cubes. But, Cubes aren’t a good design choice for a hybrid CNC/Printer. They’re great for a printer — but terrible for a mill. How many CNC mills today are cubes? Not to say they can’t be enclosed by cubes — some VMC’s like the HAAS are enclosed in a cube — but the motion mechanism is not cube based. Why?

The problem is the workpiece. CNC milling, a subtractive operation, requires moving either a heavy weight object around, or moving a high-force cutter around. Let’s say I take a block of aluminum and put it inside a cube. What’s my problems?

1. I can’t get my mounts inside the cube as well. I need some way to hold down the aluminum block, and a vice, or clamps, or even a parallel block and some bars needs space. If I need to do anything complex — say a 2 setup machining operation, then I need to re-orient the feedstock. HArd to do when the sides of the cube are blocking access. A real VMC will have the entire cube enclosure remove out of the way, specifically for setups. Usually, a real VMC or CNC mill will have a large quill that moves in Z, and a table that moves in X and Y. It’s the only way to get the accuracy a real CNC requires.

2. There’s the weight of the feedstock and vice to consider. Let’ say I have a cube design. There’s essentially 2 ways to design a cube — I move the machine tool head in X/Y and I move the feedstock in Z, or I move the tool hed in X, Y, and Z. If I need to move the feedstock in Z, I need to over-constrain the Z axis a lot — I need to handle the X/Y forces on the feedstock. If I move the Toolhead in X, Y, and Z, in a cube, then I need to overconstrain the head — I need to handle the same forces as I did when I moved Z, but now on the toolhead. What does this overconstrain mean? It means that the Z motion stage will bind very easily. It is over-constrained, and there are only a few ways to solve over-constrained linear motions systems: (a) Don’t over-constrain. Have a fixed axis and float the other parts of the axis against the fixed section. Works only if forces are low — like in a 3D printer. (b) Overconstrain, and make the linear sections of the axis perfectly linear. This is nearly impossible as a machining challenge — steel rods just aren’t that straight, and even if you got 4 steel rods that are initially straight enough, they won’t remain straight over the long term. There’s other problems, too. If you move the feedstock, then your motors have to be beefy enough to lift the maximum weight of the feedstock, vices, parallel bars, and other parts of the setup. And you have to make sure these beefy motors are perfectly in sink across all over-constrained sections. An almost impossible control task( belts stretch. Gears have backlash. Screws have unstable sections. Overconstrained designs require tolerances most machinists simply cannot achieve in the real world. You’d have to control the process of how the steel was forged — to ensure the crystal structure has no internal stress points… Possible to do — at very large scales. Like millions of units. Nearly impossible at the 100-200 unit space modern CNC machines sales hit. (c) You can solve the problem by moving the toolead in X/Z, and the work-piece in Y. This solution works really well, but it makes the size of the workpiece really small. Your Y motion platform needs 3x the space to work, and the machine gets a terrible size/workpiece ratio.

So, if you have a cube design, then you have a design failure point that every machinist knows about — the overconstrained Z axis. If you don’t know about this problem, you learn fast. You can make a machine or two work with this design flaw — it’s easy enough to keep ordering screws and rods and shimming them into perfect alignment for an Axis by hand on one machine. It may take you a week or two of time to do it — but even a month of time tuning a prototype is not a big deal. But 100 machines? You’d never ship.

So, I’ve been seeing these cool new generation of hybrid machines out there, and not bothering with them. I designed a hybrid, and I learned this problem first hand. I don’t expect most of the hybrid cube designs I see out there right now to be able to ship in large quantity — they might be able to ship a few, but managing the design flaw inherent in cubes for CNC milling is a tough problem. That’s why most VMC and Mills aren’t cube motion platforms — and the few that are are have tiny work areas. I don’t expect most of this cube generation hybrid machines to ever ship. They’ll design a machine that works — in small quantity — but just can’t scale. The labor/machine is too much for production quantities, and prices would have to rise through the roof to solve that problem. It’s the same design problem that killed LumenLabs — a design that cannot scale to production quantities due to labor/unit costs with the design, and the need for machined parts with tolerances that third party suppliers just won’t be able to meet at 100 unit/mo quantity. Part reject rates will be high. Time slip month by month, with each month reducing available resources to try again — to solve a problem that even large corporations like Roland have never solved. Until… A critical failure point, like someone getting sick, or some major order getting screwed up. Then, bankruptcy. It’s the normal way a design with a scale-up flaw dies. And a scale-up flaw — people don’t see these until they try to make 10, or 100… By then, they’ve got a design they’re committed to, and can’t afford a redesign iteration on something that, “works”. ( and works really well. A prototype or two will be amazing. The designer will never convince themselves there’s a flaw — their example works great! So, it must be the suppliers fault that the second or third unit has so many problems… “I know, let’s fin a better supplier! Oh, wait — none exist. Crap. Let’s do it ourselves. Wait, the labor/unit sale is a ton of work. Let’s go heads-down on fixing this…” That’s the reasoning to death in a scale-up design flaw.)